The present invention relates to the opening of bivalve mollusks to recover the meat of an individual mollusk either alone or in association with half of the shell. Mollusks of interest are oysters, clams, mussels and scallops.
In the case of the oyster, it will close and seal its shell in the presence of a mechanical disturbance, change in light or substances that would tend to irritate the meaty portion of the oyster. When the oyster closes its shell the adductor muscle, sometimes called the eye which passes through the body to connect the two halves of the shell, contracts and pulls the two halves of the shell together. When oysters are removed from the water and they are in a closed condition, they will retain water and can survive for various periods of time depending upon the temperature at which they are stored. However, if the oyster is warm or it is handled roughly, the adductor muscle tends to relax the halves of the shell gap, water is lost, and the oyster will die. Normally a great deal of force and mechanical manipulation is required to open a fresh oyster.
When the oyster shell is opened it immediately begins to lose much of its juice and liquid with a consequent loss of weight and flavor. Therefore, oysters are eaten immediately after the shells are opened to provide the best taste experience for the consumer.
Current methods of opening oysters and other mollusks is very labor intensive and may require grinding of the shell and the use of a special knife to pry open the shell. After opening the mollusk the meat which is attached to the shell must be removed. In the case of the oyster the eye or the membrane that attaches the meat to the top and bottom shells must be cut. If the membrane is not cut properly the meat can be ruined. Since oysters are wet, sharp instruments must be used, labor costs can be high, production can be low and worker injury is common.
Other processes used to open mollusks include steaming, radiation, and cold/hot thermal shock. Steaming, however, still requires two cuts to remove the meat and the high temperature processes provide potential breeding ground for bacteria. Radiation is difficult to do due to the variation of shell thickness and size, and requires high temperature processing. It has also been suggested that a freeze thaw condition plus tumbling is possible to produce mollusk meat devoid of the shell. However, this latter technique would not permit producing mollusk on a half-shell or individually quick-frozen mollusks such as oysters in a single process. Moreover, prior art processes have resulted in mollusk meat that was of poor quality.
U.S. Pat. No. 3,007,801 discloses and claims a freeze/atmospheric thaw process, subject to mechanical freezing temperatures of -50.degree. F. or higher. After treatment, the mollusk, e.g. oyster, is then severely tumbled in a partially frozen state to completely open an expel the oyster meat. In such a severe environment, the mixture of shell and meat in the tumbler has been shown to mutilate the oyster meat.
U.S. Pat. No. 2,824,006 discloses and claims a mechanical freeze/gas burner heat cycle to open oysters. The process relies upon the thermal shock effect to achieve the opening of the oyster.
U.S. Pat. No. 4,236,276 discloses a cryogenic freeze/gas burner heat cycle to open oysters followed by the use of severe mechanical shock for separation of the meat from the shell. The oyster is sprayed with a cryogen to chill the outer surface and then by heating it is subjected to severe thermal shock.